1. Field of the Invention
The invention relates to an alpha-beta titanium-base alloy, and fastener made therefrom. The alloy is characterized by an improved combination of strength and ductility.
2. Description of the Prior Art
The most widely used titanium-base alloy is the alpha-beta alloy Ti-6Al-4V, which is used for a wide range of applications, including sheet metal components, plate products, forgings and rod and bar products. With respect to rod and bar products, this alloy has obtained wide usage in the aerospace industry for the manufacture of fasteners. For fastener applications, the mechanical property of the alloy of most concern is the shear strength. This alloy at its highest usable heat-treated strength level has a minimum of 95 ksi shear strength with the typical shear strength range being 95 to 105 ksi. This corresponds to a typical uniaxial ultimate tensile strength (UTS) of approximately 165 to 180 ksi. Because of hardenability limitations at these strength levels, the alloy is limited to use in the production of fasteners having diameters of approximately less than 0.625 inch. At greater diameters, it is difficult to heat treat the material to adequate hardenability levels for most fastener applications.
Consequently, for fastener applications wherein larger section sizes, or higher strength levels, are required, it is conventional practice to use iron- or nickel-base alloys which are known to exhibit minimum shear strength values of 125 ksi, which correspond to 220 ksi UTS. When these alloys are used instead of titanium-base alloys, however, there results a substantial weight penalty of approximately 40%. This results from the fact that iron- and nickel-base alloys are generally 0.29 to 0.31 lb/cu.; whereas, titanium-base alloys are generally 0.165 to 0.180 lb/cu.
Weight is typically an important design consideration in most aerospace applications, and therefore it is desirable to use a titanium alloy wherein heavier section sizes and/or higher strength levels may be obtained at relatively lower weight than obtained with iron- or nickel-base alloys.
It is recognized, however, that for any alloy to be used for fastener applications a minimum level of ductility is required. Specifically, for fastener applications, this is approximately 7% elongation. Consequently, a titanium-base alloy for fastener applications desirably has 220 ksi UTS, 125 ksi shear strength and 7% elongation. It is difficult to obtain accurate and reproducible values for shear strength. Consequently, it has been determined that the shear strength minimum levels required for most fastener applications are achieved with an alloy having the capability of obtaining at least 220 ksi UTS at a minimum ductility of 7% elongation.